The graph icon will only be exposed in Summary Reports and not in Detail Reports. The logic behind this is that, for Detail reports, too many information are made available.
For example, assume we are using the Sales by item report. For Detail report, if I sold ten product X items, then all the information regarding each product sold will be available. On the other hand, for Summary report, only totals will be shown. This information will be graphable (e.g., I can put Item in the X-axis then either Qty. Sold or Total Revenue in the Y-axis).
Note: Sales by item report is only used as an example. This is applicable to all standard Sales Orders reports and Sales reports.